The present invention relates to processes and apparatus for the purification of carbon dioxide. In particular, the invention relates to the removal of impurities that are less volatile than carbon dioxide from crude carbon dioxide comprising at least one such impurity by distillation at sub-ambient temperatures and super-atmospheric pressures. The invention has particular application to the removal of hydrogen sulfide from carbon dioxide.
Carbon dioxide from naturally occurring carbon dioxide sources, such as natural carbon dioxide fields and natural gas deposits, is used for enhanced oil recovery (EOR) in some areas of the world. Some of these sources contain hydrogen sulfide, which is undesirable for pipeline transport since hydrogen sulfide is toxic and corrosive in the presence of water. In addition, it is not desirable to introduce hydrogen sulfide to the crude oil that is being extracted by the EOR process.
Processes for the removal of hydrogen sulfide from carbon dioxide are known. For example, U.S. Pat. No. 3,417,572A (Pryor, 1968) discloses a method of treating hydrogen-rich gas comprising carbon dioxide and hydrogen sulfide. The hydrogen sulfide and carbon dioxide are condensed and separated from the hydrogen-rich gas. The condensed gases are then fed to a distillation column for separation into an essentially hydrogen sulfide-free carbon dioxide overhead vapor and a bottoms liquid containing at least 10 vol. % hydrogen sulfide. The separated hydrogen-rich gas is scrubbed to remove any residual carbon dioxide and hydrogen sulfide which is then also fed to the distillation column. Overhead vapor is condensed using an external closed cycle of propane refrigerant and bottoms liquid is re-boiled using process cooling water. The distillation column has 100 trays and operates at about 590 psia (˜41 bar) so that the overhead temperature is 42° F. (˜6° C.) and the bottom temperature is about 45° F. (˜7° C.).
U.S. Pat. No. 3,643,451A (Foucar, 1972) discloses a method of producing high purity, high pressure carbon dioxide from a concentrated low pressure mixture of acid gases. The gaseous mixture is compressed, cooled and condensed and fed to a distillation column where it is separated into a high purity (at least 99.95%) carbon dioxide overhead vapor and a bottoms liquid containing condensed sulfur-containing gases. The overhead vapor is condensed using an external closed cycle of ammonia refrigerant and refrigeration duty for cooling and condensing the feed is provided by vaporizing bottoms liquid, carbon dioxide overhead liquid and the external refrigerant. The distillation column system operates at about 300 to 350 psia (˜21 to 24 bar) so that the overhead temperature is −5 to −10° F. (˜−21 to −24° C.) and the bottoms temperature is 40 to 70° F. (˜5 to 21° C.). A bottoms product of 97% hydrogen sulfide is produced in the example.
WO81/02291A (Schuftan, 1981) discloses a method for separating a gas mixture comprising carbon dioxide, at least one gas having a lower boiling point than carbon dioxide and at least one impurity (typically hydrogen sulfide) having a higher boiling point than carbon dioxide. The gas mixture is cooled and distilled in a first column to a product gas free of the impurity and a liquid fraction containing the impurity. Pure carbon dioxide is obtained in a second distillation column, which operates slightly above the triple point pressure (˜518 kPa) of carbon dioxide. Liquid product from the first column is flashed at an intermediate pressure to remove dissolved light impurities, then further reduced in pressure and evaporated before being fed to the second column as vapor. The carbon dioxide overhead vapor is practically free of impurities and the bottoms liquid fraction is rich in impurities, typically containing sulfur compounds (primarily hydrogen sulfide) at a purity of up to 50 vol. %. Reflux and re-boil are effected by a heat pump cycle which uses purified carbon dioxide as the working fluid. The working fluid is passed through a compressor, a heat exchanger and a re-boiler immersed in the bottoms liquid, where it is condensed before being fed back to the top of the column as reflux. A substantially pure carbon dioxide product is withdrawn from the circulating carbon dioxide immediately upstream of the compressor at a pressure of about 5 atm. and at near ambient temperature.